The separation of air has long been practiced using adsorption and cryogenic techniques. These techniques have been successful in recovering large percentages of the constituents of air, namely; nitrogen and oxygen. However, adsorption and cryogenic techniques are highly energy intensive because of the requirements of high pressure and extremely low temperature, respectively.
It has been known to separate oxygen from nitrogen with various chemical media. Numerous chemical interactions of oxygen from air are known to occur in a nonreversible manner. Additional chemical reactions with oxygen from air have been known to occur in a reversible manner. However, most of these reversible interactions of chemical agents with oxygen are sufficient to recover only a relatively small percentage of the oxygen, or the recovery techniques are less than desirable for a commercial continuous operation. The use of alkali metal nitrites and nitrates in combination have been known to provide a viable alternative for the separation of oxygen from air in a reversible chemical reaction.
In U.S. Pat. No. 4,132,766, a process for the separation of oxygen from air in a regenerative chemical process is set forth. Air is contacted with a molten alkali metal salt, which constitutes an oxygen acceptor. The oxidized oxygen acceptor is removed and reduced in pressure to regenerate free oxygen from the acceptor. The regenerated acceptor can then be recycled for further renewed oxidation.
In U.S. Pat. No. 4,287,170, a process is set forth for the recovery of nitrogen and oxygen separately from air. The process utilizes a first absorption and desorption cycle in which oxygen oxidizes a molten alkali metal salt mixture, and the salt mixture is subsequently depressurized to drive off the recovered oxygen. The effluent from the absorption zone, which contains residual oxygen, is then passed to a second absorption zone in which residual oxygen is removed by contact with a scavenger, such as manganese oxide. The effluent from this absorption zone constitutes a commercially pure nitrogen product. The oxidized scavenger is reduced by contact with a reducing gas which produces an exhaust effluent containing residual oxygen and reducing gas in a mixture as a by-product of impure composition.
U.S. Pat. No. 4,340,578 discloses another process for the separation of air into oxygen and an oxygen-depleted stream, wherein air is compressed, heated and contacted with an alkali metal salt mixture to oxidize the salt mixture and produce an oxygen-depleted effluent stream. The oxidized salt mixture is then depressurized in a desorption zone to recover an oxygen product. The regenerated salt mixture is recycled to the absorber or contact zone. The effluent from the absorption zone, which constitutes oxygen-depleted gas is then combusted with fuel and expanded and heat exchanged against process streams in order to provide the necessary heat for the reactions and the necessary power for the compression of feed air and product oxygen. The patent indicates that multiple stages of absorption and desorption are contemplated, but the exact flowscheme for such stages of absorption and desorption are not set forth.
The present invention provides an improved process for the recovery of oxygen from air using the molten alkali metal salt mixtures of the above prior art, wherein a greater recovery of high pressure oxygen is achieved from that of the prior art utilizing a unique system of parallel desorption which achieves an improved energy efficiency at lower capital cost as set forth below.